1. Field of the Invention
This invention relates to the use of supersensitizers in photographic emulsions.
2. Background of the Art
In most uses of silver halide in photographic materials, it is desirable to increase the speed or sensitivity of the emulsion. There are a number of different technique for increasing the speed of an emulsion which are usually classified as chemical sensitization or spectral sensitization. Chemical sensitization usually involves modification of the silver halide grains to make the most efficient use of the radiation that they absorb. The three general types of chemical sensitization are sulfur sensitization, reduction sensitization, and precious (noble) metal sensitization. These methods of chemical sensitization are well known and firmly established in the art (e.g., James, T. H. and Vanselow, W. "Chemical Sensitization", J. Photo. Sci. 1, 133 (1953), Freiser, H. and Ranz, E., Ber der Bunsengesellschaft, 68, 389 (1964), and Pouradier, J. "Chemical Sensitization", Photographic Theory: Liege Summer School, A. Hautot, p. 111, Focal Press (London) 1963).
Spectral sensitization enables grains to benefit from radiation in regions of the electromagnetic spectrum where the silver halide would ordinarily not absorb. Dyes which absorb radiation and can transfer energy to the grains to help in the photoreduction of silver ions to clusters of silver metal are conventionally used to effect spectral sensitization.
Another phenomonon associated with the use of spectral sensitizing dyes is known in the art as supersensitization. The addition of small amounts of other substances, frequently in quantities of about one percent of the molar amount of the dye, can increase the spectrally sensitized speed of the emulsion by more than an order of magnitude. Some supersensitizers are dyes themselves, but many others do not absorb radiation in significant amounts in the visible portion of the electromagnetic spectrum. Therefore, the effect of supersensitizers on spectral sensitization is not clearly dependent on the ability of compounds to absorb radiation in the visible portion of the spectrum. Certain cyanines, merocyanines compounds analogous to cyanines, certain acylmethylene derivatives of heterocyclic bases, and ketone derivatives such as p-dimethylaminobenzalacetone are known supersensitizers. Not all supersensitizers work with all spectral sensitizers. There is no general knowledge in the art that supersensitizers are capable of sensitization in the absence of spectral sensitizers. An expanded selection of supersensitizers is therefore desired.
Triphenylphosphine, P(C.sub.6 H.sub.5).sub.3 (hereinafter TPP), is reported in the literature as a supersensitizer for infrared sensitive silver halide emulsion using certain spectral sensitizing dyes (Zhurnel Nauchnio i Prikladnoi Fotografi i Kinematograffi 14 (2) 148-149 (1969) and 12 (6), 462-463 (1967), M. A. Akhmedzyanov et al. and Zh. Prikl. Spektrosk., 1980, 33 (3), 557-56 M. I. Demchuk et al.). The use of triphenylphosphine presents a number of problems because it is insoluble in water and causes dramatic variations in infrared sensitivity over moderate periods of time during coating operations.